The present invention relates to a cathode used in a gas discharge device. Specifically, the invention relates to the configuration of a cathode that extends cathode life and in turn extends the life of gas discharge devices.
Gas discharge devices are used for a number of purposes, one of which is a laser. Gas lasers are constructed to allow the excitation of a gas, such as Helium-Neon, which in turn pumps electrons into high energy states. When the electrons decay or are stimulated to fall back to their original state, the atom emits photons which make up the laser light.
One form of a laser is a ring laser used as an angular rate sensor. Ring laser angular sensors are well known in the art and are specifically described in U.S. Pat. No. 3,373,650 to Killpatrick, and U.S. Pat. No. 3,390,606 to Podgorski.
A gas discharge device generally comprises a chamber containing a gas, at least one anode and at least one cathode attached thereto. The chamber is generally formed out of a glass tube or block or the like. An electrical potential applied between the anode and the cathode causes electrons to pass through the gas filled cavity thus ionizing the gas. This ionized gas is then capable of maintaining a gas discharge, (i.e., allowing a discharge current to pass from the anode to the cathode).
In a gas discharge device ionized gas atoms (ions) are attracted to the negatively charge cathode. When these ions collide with the surface of the cathode, electrons are released creating a discharge current that passes through the gas filled cavity.
A well known problem related to cathodes used in gas discharge devices is sputtering. When high energy ions collide with the cathode surface occasionally cathode material molecules are dislodged from the cathode surface. This occurrence is known as sputtering and can adversely effect the life of the gas discharge device. This problem is particularly troublesome in gas discharge devices used as lasers since loss of gas molecules will cause the laser to be extinguished. Dislodged cathode material molecules can trap or bury gas atoms beneath the surface of the cathode. This reduces the number of gas atoms available for maintaining a gas discharge.
The gas filled cavity is generally closed having a limited amount of gas therein. When sputtering occurs, the overall amount of gas is reduced due to the gas atoms being lodged beneath the cathode surface. If sputtering continues, eventually there will not be enough gas to support the discharge. Therefore, sputtering has very adverse effects on the life of the gas discharge.
The prior art has utilized a metallic cathode (primarily aluminum) with an oxide layer placed on the electron emitting surface. This process helps to reduce sputtering because the oxide surface is generally harder than the metallic material of the cathode. However, high concentrations of ion flow (high current density) develops in particular areas on the surface of the cathode due to irregularities in that surface, thus causing the oxide layer to breakdown. The breakdown of the oxide layer causes sputtering to occur in these areas. The problem of high current density may occur in almost all cathodes whether they have an oxide layer or not.
Another approach to eliminate the problem of sputtering is to use a cathode with a curved interior surface. High current density typically occurs on portions of the cathode surface that are discontinuous (e.g. corners, sharp points). By having curved interior surfaces, the build up of high current density in particular areas of the cathode surface is reduced.
A further approach of the prior art to reduce sputtering is to embed discharge gas molecules in the cathode surface prior to sealing the cavity. This causes an exchange of discharge gas atoms as opposed to the capture of gas atoms (e.g. when a gas atom is lodged in the cathode surface, it will be likely to knock out a previously lodged gas atom.) The added number of discharge gas atoms available extends the life of the gas discharge. An example of this approach is shown in U.S. Pat. No. 4,853,940 to Ford et al.
Different geometries of the discharge cavity can effect the operation of the gas discharge. Specifically, the configuration of the cavity and the electrodes (cathode and anode) can change the electrical potential required to maintain the gas discharge. This process is known in the art as the hollow cathode effect and is described in three publications by D. J. Sturges and H. J. Oskum, J. of Applied Physics No. 35 (1964), J of Applied Physics No. 37 (1967) and Physics, No. 37 (1967).
Using hollow cathode effect, secondary electron emission is maximized. When an ion - electron interchange occurs at the surface of the cathode, an instability in the cathode material is created. This instability allows for the emission of a second electron (a secondary electron).